Crystal structures 3-form

It may be desirable to predict which crystal structure is most stable in order to predict the products formed under thermodynamic conditions. This is a very difficult task. As of yet, no completely automated way to try all possible crystal structures formed from a particular collection of elements (analogous to a molecular conformation search) has been devised. Even if such an effort were attempted, the amount of computer power necessary would be enormous. Such studies usually test a collection of likely structures, which is by no means infal-... 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Colloidal crystals . At the end of Section 2.1.4, there is a brief account of regular, crystal-like structures formed spontaneously by two differently sized populations of hard (polymeric) spheres, typically near 0.5 nm in diameter, depositing out of a colloidal solution. Binary superlattices of composition AB2 and ABn are found. Experiment has allowed phase diagrams to be constructed, showing the crystal structures formed for a fixed radius ratio of the two populations but for variable volume fractions in solution of the two populations, and a computer simulation (Eldridge et al. 1995) has been used to examine how nearly theory and experiment match up. The agreement is not bad, but there are some unexpected differences from which lessons were learned. 

The results of a crystal structure formed by a trans opening of the BPDE l(+) to yield 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydro-benzo[a]pyrene (BPTOH) shows a Cde (90) conformation of the ring. The 07-HOT and O8-H8 groups are de, and the 09-H9 and 010-H10 are also de. The torsion angles of the benzo ring are in best agreement with our second most stable structure, Cde, of the anti BDE-N2(G) trans adduct as is seen from Table III. In adduct formation to N2(G) the trans adduct is the major product (13-22) ... 

The thin films of the nylon samples that were melt-pressed for FTIR examination gave similar results. Linear samples pressed out easily to make thin films that were excellent for IR evaluation. Star branched materials would not press thin enough to give good spectra even at maximum press pressure. It was originally hoped that star and linear nylons would have different crystal structure forms (JL4) unfortunately, FTIR has shown little differences other than those that occur from differences in sample preparation (Figure 5). 

According to V. M. Goldschmidt, two substances with the same basic formula and crystal structure form solid solutions in a concentration range that depends on the degree of similarity of their ionic radii. A large range of solid solutions may be expected if the radius of the larger ion does not exceed that of the smaller by more than 15% [3.73a],... 

For example, many ionic compounds crystallize from a water solution with water molecules incorporated into their crystal structure, forming a hydrate. Hydrates have a specific number of water molecules chemically bonded to each formula unit. A chemist may know the formula of the ionic part of the hydrate but not how many water molecules are present for each formula unit. 

A (molecular) crystal polymorph is a solid crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state Conformational polymorphs are formed by molecules that can adopt different conformations in different crystal structures formed by coordination complexes where ligands bound in delocalized bonding modes adopt different relative orientations Concomitant polymorphs are polymorphic modifications of the same substance obtained from the same crystallisation process Pseudo-polymorphs are ... 

S. Mohamed, D.A. Tocher, M. Vickers, P.G. Karamertzanis, S.L. Price, Salt or cocrystal A new series of crystal structures formed from simple pyridines and carboxylic acids, Cryst. Growth Des. 9 (2009) 2881-2889. 

The application of LSR to amino-acids has received some attention. (451-456, 498) Such studies are an essential preliminary to the use of LSR for amino-acid sequence determination in simple peptides and proteins. The latter are discussed more comprehensively in Section G. A detailed study has been made (453) of the interaction of Eu(iii), Pr(iii), Gd(iii), and La(iii) with iV-acetyl-L-3-nitrotyrosine in order to characterize the nitrotyrosine residue as a potential specific lanthanide binding site in proteins. The parameters of the dipolar interaction indicate a significant contribution from non axially symmetric terms. The conformations of the nucleotides cyclic j8-adenosine 3, 5 -phosphate (3, 5 -AMP) (457, 458) and adenosine triphosphate (ATP) (459) have been deduced using LSR. In the former case the conformation of the ribose and phosphate groups is consistent with the solid state structure. A combination of lanthanide shift and relaxation reagents was used to deduce the most favoured family of conformations for ATP in aqueous solution. One of these conformations corresponds closely to one of the crystal structure forms. 

Five examples of using the ICSD database in solving crystal structure form powder diffraction data by structural analogy are discussed in J.A. Kaduk, Use of the Inorganic Crystal Structure Database as a problem solving tool, Acta Cryst. B58, 370 (2002). 

Fig. 1 a-c Schematic representation of a channel type b cage herringbone type c cage brick type, crystal structures formed by crystalline cyclodextrin inclusion complexes. (Adopted from [18] with permission)... 

Molybdenum, niobium, and tantalum (bcc crystal structure) form a continuous series of solid solutions with tungsten, but only Nb and Ta additions lead to a strong straining effect. Higher additions of Nb and Ta raise the recrystallization temperature but also increase the DBTT and thus decrease the workability. Therefore, none of these binary or ternary tungsten base alloys developed in the past [6.2] has attained commercial importance. The only important tungsten-base solid-solution alloy today is tungsten-rhenium. 

More recent studies have shown the consistency of the crystal structure of PAn/ CSA to be variable and highly dependent on the conditions of film preparation. Sara-vanan and coworkers197 have reported a crystallinity as high as 56% in CSA-doped PAn. Djurado and coworkers198 have speculated that the influence of chain branching during polymerization may play a role in the nature of the crystal structure formed. 

A. The G-quadruplex formed by d[AGGG(TTAGGG)3] is of particular interest because it represents four units of the human telomere repeat sequence. Details of the crystal structure formed by this sequence in the presence of ions and in the presence of Na ions are discussed below in Section 4.6. The crystal structure of the bimolecular G-quadruplex adopted by the dodecamer sequence d(TAGGGTTAGGGT), also derived from the human telomere repeat sequence, has been determined and similarly contains K" ions coordinated between adjacent G-quartets (PDB ID 1K8P). ... 

Figure 2 Chocolate is carefully processed so that the proper crystal structure forms in the chocolate. These crystals give chocolate the characteristics found in popular chocolate bars.

Each crystal structure forms an entry in the CSD and is identified by a CSD reference code (refcode). e.g., BAGFIT02. Six letters identify the chemical compound. 

Thermal conductivity. Phonon conductivity is most efficient in simple crystal structures formed by small atoms. 

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